Internally cooled large aperture microlens array with monolithically integrated microscanner

ABSTRACT

A large aperture microlens array assembly has at least two arrays of microlenses with individual unit cell trains optically interconnecting individual microlenses in one array with related individual microlenses in another array. In each unit cell train the light entering an entrance pupil of a microlens in one array is transmitted through the exit surface of a related microlens of the other array to provide a collimated output through the exit. One array may be moved with respect to the other array for scanning a field of regard.

CROSS REFERENCE TO RELATED U.S. APPLICATION

This is a divisional of application Ser. No. 08/011,323, filed on Jan.29, 1993, now U.S. Pat. No. 5,420,720, is a continuation-in-part of U.S.patent application Ser. No. 07/904,316 filed Jun. 25, 1992 and entitled"DISPERSIVE MICROLENS", George Gal (inventor), and assigned to the sameassignee as the assignee of this application. This application claimsthe benefit of the filing date for the subject matter which is common tothe parent application Ser. No. 07/904,316 filed Jun. 25, 1992.

This application is also a continuation-in-part of application Ser. No.07/982,514 filed Nov. 27, 1992 and entitled "METHOD AND APPARATUS FORFABRICATING MICROLENSES", now U.S. Pat. No. 5,310,623 issued on May 10,1994. George Gal (inventor), and assigned to the same assignee as theassignee of this application. This application claims the benefit of thefiling date for the subject matter which is common to the parentapplication Ser. No. 07/982,514 filed Nov. 27, 1992.

BACKGROUND OF THE INVENTION

This invention relates to a large aperture microlens array apparatus.

This invention relates particularly to a large aperture microlens arrayassembly having at least two arrays of microlenses with individual unitcell trains optically interconnecting individual microlenses in onearray with related individual microlenses in another array. In each unitcell train the light entering an entrance pupil of a microlens in onearray is transmitted through the exit pupil of a related microlens ofthe other array to provide a collimated output through the exit pupil.

SUMMARY OF THE INVENTION

A large aperture microlens array apparatus constructed in accordancewith the present invention includes a first array of microlenses, havinga plurality of microlenses disposed across the array, and a second arrayof microlenses, having a plurality of microlenses disposed across thearray. Unit cell train means optically interconnect individualmicrolenses in the first array with related individual microlenses inthe second array so that light entering an entrance pupil of a microlensin one array is transmitted through the exit pupil of a relatedmicrolens in the other array.

The microlenses and unit cell train means are constructed so that amicrolens in one of the arrays images the light entering the entrancepupil and transmits, through a related individual unit cell train, theimaged light to a related microlens in the other array. The relatedmicrolens in the other array receives the imaged light, through therelated individual unit cell train, and provides a collimated outputthrough the exit pupil.

The individual unit cell trains coact to provide a large aperture fortransmitting a beam of light or a multiplicity of beams of light as acomposite or composites of the light beam segments transmitted throughthe individual unit cell trains over a field of view of the aperture.

The arrays of microlenses are constructed to form an afocal largeaperture in which light can be transmitted in either direction throughthe arrays of microlenses.

In certain embodiments of the invention, beam-shaping means are disposedadjacent the exit pupil side of the aperture for shaping the beam oflight transmitted through the large aperture.

In one embodiment of the invention a telescope is positioned adjacentthe entrance pupil side of the aperture. The telescope reduces thediameter of the beam of light, as transmitted through the telescope tothe aperture, so that a smaller size (diameter) of the arrays ofmicrolenses can be used to scan a particular aperture field of regardthan the aperture field of regard possible without the telescope.

The construction of the microlenses in the arrays provides opticallyinactive volumes with respect to the individual microlenses in eacharray. The optically inactive volumes can be used for functions otherthan transmitting a beam of light through a large aperture.

In a certain embodiment of the invention cooling channels, for thecirculation of cooling fluid, are fabricated in the optically inactivevolumes, within at least one of the microlens arrays, for removing heattransmitted into or generated within that microlens array. The coolingchannels can be fabricated within all of the arrays.

In one embodiment of the invention the first and second arrays ofmicrolenses are mounted in a static, fixed relationship to one another.

In another embodiment of the invention one of the arrays is mounted fordynamic movement with respect to the other array. The movement of onearray with respect to the other array permits scanning and viewing of alarger aperture field of regard than the aperture field of view possiblewith a static, fixed mounting of the two arrays.

Array positioning and control apparatus change the position of one arraywith respect to the other and control the exact position of one arraywith respect to the other array.

In certain embodiments of the invention the light transmitted throughthe large aperture is monochromatic light of a certain frequency, andthe movable array is positionable at a plurality of Eigen positions withrespect to the frequency of the monochromatic beam of light to provideimproved diffraction point spread function response and, in some cases,to obtain satisfactory image quality.

In certain embodiments of the invention each individual unit cell trainprovides a minus one (-1) magnification of the exit pupil as an image ofthe entrance pupil. The minus one (-1) magnification can be achievedwith just two arrays of microlenses. The minus one (-1) magnificationreferred is a minus one (-1) magnification as can be achieved withindesign and engineering tolerances and might in fact vary by somepercentage.

In several embodiments of the invention the first array comprisesindividual microlenses which function as wide field static imagers ineach related unit cell train, and the second array comprises individualmicrolenses which function as collimating scanners in each relatedindividual unit cell train.

In certain embodiments of the present invention the movable array ismovable over an arc to provide the collimating scanner function.

In certain embodiments of the invention each unit cell train has arelated microlens in the first array which is constructed to form thelight entering the entrance pupil of that microlens into an internalimage within that microlens, and the entrance pupil serves as theaperture stop for the unit cell train.

In one embodiment of the invention in which the unit cell trains producea minus one (-1) magnification and in which only two arrays ofmicrolenses are used, the second array is movable with respect to thefirst array to provide scanning.

In this embodiment each microlens in each array has two surfaces.

In this embodiment the front surface of each microlens in the firstarray (which forms the entrance pupil) is constructed as a generalaspheric surface. This first, general aspheric surface may be arotationally symmetric surface. This surface may be a non-rotationallysymmetric surface for operation at a look angle which is biased withrespect to a normal of the surface of the large aperture.

The second surface of each microlens in the first array is a generalaspheric surface.

Each unit cell train includes an air space within the relatedmicrolenses in the first and second arrays; and the second, generalaspheric surface of a microlens in the first array renders the air spacetelecentric and corrects coma in the unit cell train.

The third surface of a unit cell train is the inlet surface on amicrolens of the second array and is a conic aspheric surface to formthe outlet pupil on the fourth, exit pupil surface of the microlens inthe second array.

The fourth, exit pupil outlet surface on a microlens in the second arrayis a general aspheric surface which collimates the output, correctsaspherical aberration and functions as an exit pupil at minus one (-1)magnification.

In one embodiment of the invention all the surfaces on the microlensesin a unit cell train are refractive surfaces.

In another embodiment of the invention at least some of the surfaces ina unit cell train are diffractive surfaces or a combination ofdiffractive and refractive surfaces.

In another embodiment of the invention the microlens array assemblycomprises more than two arrays of microlenses and provides a plus one(+1) magnification at the outlet of the aperture. Each individual unitcell train in this embodiment has three pupils comprising the entrancepupil, the exit pupil, and an internal pupil; and each unit cell trainforms two internal images.

Another embodiment of the invention provides a fabrication process forforming cooling channels in silicon used for the microlens arrayassembly. This process includes a fusion bonding between two blocks ofsilicon, after the cooling channels have been etched into one of theblocks, and includes a subsequent fabrication of microlenses on theouter surfaces of the two fused blocks of silicon. The fabrication ofthe microlenses may be produced by binary etching or preferably by grayscale fabrication as disclosed in pending U.S. application Ser. No.07/982,514 filed Nov. 27, 1992 and entitled "METHOD AND APPARATUS FORFABRICATING MICROLENSES", George Gal (inventor) and assigned to the sameassignee as the assignee of this application. As noted above, thisco-pending application Ser. No. 07/982,514 is a parent application ofthis application; and the application Ser. No. 07/982,514 isincorporated by reference in this application.

The co-pending application Ser. No. 07/904,316 filed Jun. 25, 1992,entitled "DISPERSIVE MICROLENSES", George Gal (inventor), and which isalso noted above as a parent application of this application, disclosesbinary techniques for fabricating arrays of microlenses havingrefractive surfaces and having diffractive surfaces. The applicationSer. No. 07/904,316 filed Jun. 25, 1992 is incorporated by reference inthis application.

Methods and apparatus which incorporate the features described above andwhich are effective to function as described above constitute further,specific objects of the invention.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings, which by way of illustration, show preferredembodiments of the present invention and the principles thereof and whatare now considered to be the best modes contemplated for applying theseprinciples. Other embodiments of the invention embodying the same orequivalent principles may be used and structural changes may be made asdesired by those skilled in the art without departing from the presentinvention and the purview of the appended claims.

BRIEF DESCRIPTION OF THE DRAWING VIEWS

FIG. 1 is an isometric view of a large aperture microlens arrayapparatus constructed in accordance with one embodiment of the presentinvention. FIG. 1 is partly broken away and shown in cross sectionthrough the arrays of microlenses. The cross section shows how thearrays are constructed to create optically inactive volumes in thearrays.

FIG. 2 is an enlarged, fragmentary view in cross section showing detailsof the structure encircled by the arrows 2--2 in FIG. 1. FIG. 2 showsoptical unit cell trains between related microlenses in the two arraysand shows how micro cooling channels are positioned within the opticallyinactive volumes in the arrays of the microlenses.

FIGS. 3-6 are isometric views showing how a sequence of fabricationsteps are used for forming the cooling channels in the array materialand then forming the microlenses on the surfaces of the array materialof the microlens arrays shown in FIGS. 1 and 2.

FIG. 3 shows how cooling channel grooves are etched into the arraymaterial.

FIG. 4 shows how a plate of array material is fusion bonded to the blockof array material in which the grooves have been etched.

FIG. 5 shows how the fusion bonded assembly is trimmed and polished onboth outer surfaces.

FIG. 6 shows how the microlenses are formed on the outer surfaces whichhad been trimmed and polished. The surfaces of the microlenses shown inFIG. 6 can be formed by binary etching or by gray scale etching.

FIG. 7 is a fragmentary, enlarged, isometric view, partly broken awayand in cross section, showing the construction and the dynamics ofoperation of a single, unit cell train and the two related microlensesin the microlens array assembly shown in FIGS. 1 and 2. FIG. 7 shows howthe light entering an entrance pupil in the upper (as viewed in FIG. 7)microlens in one of the arrays is formed into an internal image withinthe unit cell train and is transmitted out of an exit surface in thelower (as viewed in FIG. 7) microlens. FIG. 7 illustrates how theforming of the entering light to an internal image provides opticallyinactive volumes within the arrays of microlenses and shows how theoptically inactive volumes are, in this case, utilized for coolingchannels.

FIG. 8 is a fragmentary, enlarged view in cross section taken along theline and in the direction indicated by the arrows 8--8 in FIG. 9.

FIG. 9 is a fragmentary, enlarged view in cross section taken along theline and in the direction indicated by the arrows 9--9 in FIG. 1 andFIG. 2.

FIGS. 8 and 9 show details of both the micro cooling channels and themacro cooling channels for circulating cooling fluid in the microlensarrays shown in FIGS. 1 and 2.

FIG. 10 is a isometric view of a large aperture microlens arrayapparatus, like FIG. 1, but showing another embodiment of the presentinvention. In the embodiment shown in FIG. 10, one of the arrays ofmicrolenses is mounted for dynamic movement with respect to the otherarray to permit scanning and viewing of a larger aperture field ofregard than the aperture field of view possible with a static, fixedmounting of the two arrays.

FIGS. 11-16 are side elevation views of a unit cell train used in theFIG. 10 embodiment and comprising one microlens in one array and arelated microlens in the other array.

FIG. 11 shows how the second, collimating scanner microlens in the unitcell train (the microlens on the right hand side of FIGS. 11-16) scansupwardly and downwardly over an arc with respect to the first, widefield static imager microlens (the microlens on the left hand side ofFIGS. 11-16) in the cell train. The extent of the upward and downwardscanning movement is indicated in FIG. 11 by the solid line and dashedline positions of the collimating scanner microlens. It should be notedthat the scan is achieved in a single direction (as drawn in FIG. 11) orin a direction normal to that illustrated, or in both directionssimultaneously.

Each of FIGS. 11-16 trace representative rays of the light transmittedthrough the unit cell train.

FIGS. 12-16 show the collimating scanner microlens positioned atrespective 0°, 5°, 10°, 15° and 20° angles with respect to the widefield static imager microlens.

FIG. 17 is a series of views showing the blur spots produced when thelight transmitted out of the unit cell train shown in FIG. 12 is broughtto a focus by a beam-shaping lens assembly. The blur spots shown in FIG.17 are the blur spots which are produced by one individual unit celltrain looking at the different instantaneous fields of view (IFOV)indicated by the respective cross scan and along scan coordinatelocations shown in FIG. 17 with the collimating scanner element of theunit cell train in the 0° scan position shown in FIG. 12. The blur spotsshown in FIG. 17 are blur spots produced by monochromatic light. ThisFIG. 17 shows the quality of the image over a 4° field of view as viewedby one cell train at a fixed position (not scanned) of 0°.

FIG. 18 is a view like FIG. 17 but showing the blur spots produced by anindividual unit cell train positioned at the 20° angle of scan of thecollimating scanner element as shown in FIG. 16. FIGS. 17 and 18demonstrate the high degree of optical correction and image quality overa substantial (4° by 4°) FOV at every FOR to plus or minus 20°.

FIG. 19 is a cross section view through the two microlens arrays of theembodiment shown in FIG. 10 with the collimating scanner elements at a0° scan angle. FIG. 19 traces representative rays through the individualunit cell trains for a 0° angle of scan and illustrates how the formingof the internal image provides optically inactive volumes in the arrays.The two microlens arrays shown in FIGS. 10-19 provide a minus 1 (-1)magnification at the exit pupil. FIG. 19 also shows how cooling channelsare incorporated within the optically inactive volumes in one of thearrays (the array on the left hand side as viewed in FIG. 19).

FIG. 20 is a computer generated, isometric view showing the mosaic ofwavefronts just after the exit pupil surface of a 9×9 array of unitcells at the 0° angle of scan shown in FIG. 19. At a 0° scan angle andwith monochromatic light the minus one (-1) magnification unit celltrains shown in the FIG. 19 arrays produce a composite wavefront at theoutlet of the large aperture in which there is no offset between theadjacent wavefronts at the exit pupils of the individual unit celltrains.

FIG. 21 is a computer-generated, isometric view correlated to FIG. 20and showing the monochromatic diffraction point spread function at thefocal plane of an imaging telescope for the arrays of microlenses shownin the FIG. 10 embodiment at a 0° scan as shown in FIG. 19 and formonochromatic light.

FIG. 22 is a view like FIG. 19 but shows the collimating scannermicrolenses in the scanning array positioned at a 20° angle of scan.FIG. 22 illustrates how, at this large angle of scan, the lighttransmitted through the individual unit cell trains is imaged to provideoptically inactive volumes which are sufficiently large enough for theinclusion of the cooling channels without any interference with theoptical performance of the individual unit cell trains.

FIG. 22 also illustrates how, with the minus 1 (-1) magnificationconstruction, an incoming wavefront (illustrated by the line normal tothe ray traces in front of the entrance pupils) becomes offset in thesegments of the composite light beam transmitted through the exit pupilsof the individual unit cell trains. The amount of the offset (an offsetin phase from one segment to another segment) depends upon thewavelength of the monochromatic light being transmitted through themicrolens arrays and the physical spacing between the unit cell centersand the angle of incidence of the incoming wavefront to the array. Theoffsets occur because of the differences in the lengths of the opticalpaths from an incoming wavefront through adjacent cell trains. In thiscase, diffraction point spread function formation occurs ideally whenthere is an integer number of wavelengths from step to step in theoffsets. This condition occurs at a two dimensional (x,y) sequence ofdiscrete angles which will be referred to as Eigen angles.

FIG. 23 is a view like FIG. 20 but showing the offset of the individualwavefronts transmitted through the individual exit pupils at the 5°angle of scan shown in FIG. 13.

FIG. 24 is a view like FIG. 21 but showing how the point spread responseis distributed into a series of spaced apart peaks of varying magnitudeat the focal plane for the 5° angle of scan shown in FIG. 13 and formonochromatic light of a 4.0 micrometer wavelength. This 5° angle ofscan was not substantially an Eigen angle (or in close proximity to anEigen angle) for the particular wavelength of the monochromatic lighttransmitted through the arrays.

FIG. 25 is a view like FIG. 20 but showing the offsets for imagestransmitted through the individual unit cell trains in two 9×9 microlensarrays having the minus 1 (-1) magnification construction shown in FIG.10 with a 5° angle of scan as shown in FIG. 13 and operating withmonochromatic light at a wavelength of 3.775 micro meters.

FIG. 26 is a view like FIG. 21 but showing the monochromatic diffractionpoint spread function for the 5° angle of scan shown in FIG. 13. This 5°angle of scan was substantially an Eigen angle for the particular 3.775micrometer wavelength of monochromatic light transmitted through thearrays.

FIG. 26 illustrates how a point source can be resolved, but an extendedimage (over a substantial field of view) is not satisfactorily resolvedwith minus one (-1) magnification under these circumstances.

FIG. 27 is a side elevation view showing unit cell trains in a microlensarray assembly constructed in accordance with another embodiment of thepresent invention. The unit cell trains in this embodiment provide plusone (+1) magnification. The embodiment shown in FIG. 27 comprises fourarrays of microlenses (rather than just the two arrays of microlensesshown in the minus one (-1) magnification cell train embodiments shownin FIGS. 1 and 10). In FIG. 27 the back to back arrays in the center ofthe unit cell trains are movable with respect to the outer arrays toprovide scanning. FIG. 27 shows all of the elements of a unit cell trainaligned for a 0° angle of scan, and FIG. 27 traces representative raysthrough the unit cell train for this 0° angle of scan.

FIG. 28 is a view like FIG. 27 but traces representative light raysthrough the unit cell train for a plus 5° angle of scan and for a minus5° angle of scan.

FIG. 29 is a view like FIG. 20 but shows the offset of the wavefrontsfrom the respective individual unit cell trains in a 9×9 array ofmicrolenses for an offset of the scanning arrays of 3.0 micro meters forthe plus one (+1) unit cell train embodiment shown in FIGS. 27 and 28.This particular scanning offset position was not an Eigen position forthe particular wavelength of the monochromatic light transmitted throughthe individual unit cell trains.

FIG. 30 is a view like FIG. 21 showing the monochromatic diffractionpoint spread function corresponding to the offsets in the wavefronts asillustrated in FIG. 29.

FIG. 31 is a view like FIG. 20 showing offsets of the images transmittedthrough the individual cell trains of a 9×9 microlens array having theplus one (+1) magnification unit cell train construction illustrated inFIGS. 27 and 28, but for an offset equal to 9.5 micro meters.

FIG. 32 is a view like FIG. 21 but showing the monochromatic diffractionpoint spread function for the 9.5 micro meter offset illustrated in FIG.31. In this case the 9.5 micrometer offset position of the scannerelement was an Eigen position with respect to the wavelength of themonochromatic light transmitted through the four arrays of microlenses.

FIG. 32 illustrates how a point source can be resolved, but asubstantially extended FOV image is also satisfactorily resolved with aplus one (+1) magnification under monochromatic circumstances.

FIG. 33 is a side elevation view showing how, in accordance with anotherembodiment of the invention, a telescope can be positioned adjacent theentrance side of the large aperture. The telescope reduces the diameterof the light beam transmitted through the telescope. This enables asmaller size (diameter) of the arrays of microlenses to be used forviewing a particular field of regard, using the scanning movement of themovable array of the microlenses, than would be possible without thetelescope.

FIG. 34 is a view listing the optical design parameters (prescription)for the unit cell train shown in FIG. 11 with one millimeter squaremicrolenses constructed of silicon having an index of refraction of3.426963.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of a large aperture microlens array apparatus constructedin accordance with the present invention is indicated generally by thereference numeral 41 in FIG. 1.

In this embodiment the apparatus 41 includes a microlens array assembly43, a beam shaping lens assembly 45, and a focal plane 47.

A detector 49 (which may be a monochromatic or a polychromatic detector)is located at the focal plane for detecting light transmitted throughthe apparatus 41 from the left to the right as viewed in FIG. 1. Thedetector 49 has individual pixels 50.

The large aperture provided by the microlens array assembly 43 is afocaland light can also be transmitted through the apparatus 41 from theright to the left as viewed in FIG. 1.

A light source, such as, for example, a monochromatic laser 51, can bemounted at the focal plane in place of or in addition to the detector49; and the laser acts as a light source for projecting light throughthe apparatus 41.

The reference numerals 53, 54, 55 and 57 trace representative light raysas transmitted through the apparatus 41. This will be described in moredetail below with particular reference to FIG. 7.

As shown in FIG. 1 and FIG. 2, the microlens array assembly 43 comprisestwo arrays 43A and 43B of microlenses mounted in fixed, staticrelationship to one another.

The first array 43A of microlenses comprises individual microlenses 61,each having an entrance pupil 65 (when light is transmitted through theapparatus 41 from left to right as viewed in FIG. 1). The second arrayof microlenses comprises individual microlenses 63, each of which has anexit surface 67 (when light is transmitted through the apparatus 41 fromleft to right as viewed in FIG. 1).

A unit cell train comprising an individual microlens 61 in one array anda related individual microlens 63 in the other array is indicated by thereference numeral 68 in FIG. 7.

As light is transmitted through the apparatus 41 (from left to right asviewed in FIG. 1 and from top to bottom as viewed in FIG. 7) an incomingray 53 is refracted as it passes through the refractive surface on thepupil 61 and travels through the first microlens at a particular angle,as indicated by the reference numeral 54. All of the light incoming toan individual microlens is therefore imaged at 56, and the image is thentransmitted to the exit pupil 67 of a related microlens 63. The lightray exits as 55 at the same angle (with respect to the other light raysin the bundle, or segment, of light rays transmitted through theindividual unit cell train provided by the related microlenses 61 and63) as the angle at which that light ray entered the microlens arrayassembly 43. All of the light entering an entrance pupil 65 istransmitted out of the exit surface 67 when the assembly 43 isperpendicular to the incoming light as illustrated in FIG. 1 withoutyignetting for a substantially narrow field of view.

FIG. 7 shows how an incoming ray 53 is refracted and follows a path 54traveling through the microlenses of the unit cell train 67 and thenexits as the ray 55.

The individual unit cell trains 68 in the microlens array assembly 43coact to provide a large aperture for transmitting a beam of light as acomposite of the individual light beam segments transmitted through theindividual unit cell trains in the microlens array assembly 43.

The light beam segments transmitted through the individual unit celltrains are combined at the outlet of the large aperture provided by themicrolens array assembly 43, and the combining of these light beamsegments produces a composite beam of light at the outlet of the arraysubstantially corresponding to the incoming beam of light entering themicrolens array assembly 43.

In the static, fixed mounting of the two arrays of microlenses in theassembly 43 as illustrated in FIGS. 1 and 2, the individual unit celltrains each provide a minus one (-1) magnification.

The mechanism of operation of a unit cell train and the way in which aunit cell train collimates the light at an exit pupil with respect tothe light at the entrance pupil will be described in more detail belowwith particular reference to FIGS. 11-16, 19 and 22.

Because the light at an exit pupil of a cell train is collimated, as isthe light at the entrance pupil, the microlens assembly 43 provides anafocal optical design. Light is permitted to be transmitted in eitherdirection through the apparatus 41.

When a light source, such as, for example, a laser 51, is used toproject light in a direction from right to left (as viewed in FIG. 1),the light incoming to the microlens array assembly 43 is represented bythe reference numeral 55 and the outgoing light is represented by thereference numeral 53.

It is a very important feature of the present invention that themicrolens array assembly 43 forms an internal image 56 in eachindividual unit cell train to provide optically inactive volumes 59 inthe microlens array assembly 43.

These optically inactive volumes can be used for non-optical functions.

In the embodiment of the invention illustrated in FIGS. 1 and 2, theoptically inactive volumes 43 are used for the construction andoperation of micro cooling channels 69.

As best illustrated in FIGS. 8 and 9, each micro cooling channel 69 isconnected to macro cooling channels, or manifolds, 71. A pump 73circulates cooling fluid from a tank or reservoir 75. A heat exchangeror radiator 79 is connected in series with the tank 75 and the pump 73for radiating or otherwise removing heat from the cooling fluidcirculated through the cooling channels 69.

The heat removed from the microlens array assembly 43 can be heat whichis transmitted into the array, or it can be heat which is generatedwithin the microlens array assembly.

Thus, for example, if a relatively high powered laser 51 is used as alight projector, the laser light itself, in passing through themicrolens array assembly 43, can generate heat within the material ofthe microlenses making up the arrays.

By way of further example cooling may be required to reduce the selfemission of heat for sensor sensitivity requirements.

FIGS. 3-6 are isometric views showing a sequence of fabrication stepsused for forming the cooling channels 69 in the in the silicon materialof the assembly 43.

These FIGS. 3-6 also show how the microlenses are then formed on thesurfaces of the silicon material, after the fabrication of the coolingchannels 69.

As shown in FIG. 3, the grooves for the cooling channels 69 are etched,by photolithography etching, into an upper surface of a first block 75of silicon.

As shown in FIG. 4, a second block 77 of silicon is then fusion bondedto the block 75 at the surface at which the cooling channels 69 havebeen etched.

As shown in FIG. 5, the top and bottom surfaces 79 and 81 of the fusedtogether blocks are then trimmed and polished.

Finally, as shown in FIG. 6, the first array 43A and the second array43B of microlenses 61 and 63 are then formed on the respective top andbottom surfaces which had previously been trimmed and polished.

The arrays 43A and 43B of the microlenses 61 and 63 can be fabricatedeither by binary techniques or by the gray scale technique described inpending U.S. patent application Ser. No. 07/982,514 filed Nov. 27, 1992in the U.S. Patent and Trademark Office by George Gal, inventor, andentitled "METHOD AND APPARATUS FOR FABRICATING MICROLENSES" and assignedto the same assignee and the assignee of this application. Theapplication Ser. No. 07/982,514 is incorporated by reference in thisapplication.

In a specific embodiment of the present invention in which themicrolenses were 1 millimeter square microlenses, the grooves 69 wereetched to a 300 micrometer depth and a 200 micrometer width. The exactcooling channel dimensions are design options.

Another embodiment of the large aperture microlens array apparatus 41 isshown in FIG. 10.

In the FIG. 10 embodiment, parts which correspond to parts shown in theFIG. 1 embodiment have been indicated by the same reference numerals.

In the embodiment shown in FIG. 10, one array (the array 43B) ofmicrolenses is mounted for dynamic movement with respect to the otherarray (the array 43A) of microlenses. This permits scanning and viewingof a larger aperture field of regard than the aperture field of viewpossible with the static, fixed mounting of the two arrays 43A and 43Bof the FIG. 1 embodiment.

In FIG. 10 the field of view (the field which can be viewed without anyscanning) is indicated by the arrow designated "Field of View (FOV)".The FOV would typically be defined by the overall dimensions of an arraydetector, such as, the detector 49.

The field of regard which can be viewed by scanning of the array 43B isindicated by the legend "Field of Regard (FOR)".

The instantaneous field of view which can be viewed by an individualunit cell train in the array is indicated by the legend "IFOV(Instantaneous Field of View)" in FIG. 10. The IFOV would typically bedefined by the dimensions of a single pixel 50 in the array detector 49.

The overall microlens array assembly 43 thus provides an agile beamsteering unit, as indicated by the corresponding legend in FIG. 10.

FIGS. 11-16, 19 and 22 show further details of the arrays 43A and 43B inthe FIG. 10 embodiment and further details of the microlenses 61 and 63within those respective arrays 43A and 43B.

FIG. 34 lists the prescription for the cell train 68 shown in FIG. 11and comprising one millimeter square microlenses constructed of siliconhaving an index of refraction of 3.426963.

As shown in FIG. 11, each individual unit cell train 68 comprises amicrolens 61 which is a wide field static imager and a microlens 63which is a collimating scanner.

The collimating scanner 63 scans over a suitable arc (not a plane) andis movable between the solid line position and the dashed line positionshown in FIG. 11. It should be noted that the scan is achieved in asingle direction (as drawn in FIG. 11) or in a direction normal to thatillustrated, or in both directions simultaneously.

As best shown in FIGS. 11-16, each individual unit cell train 68 in theFIG. 10 embodiment includes an air space 83 between the facing surfacesof the wide field static imager 61 and the collimating scanner 63.

Each unit cell train 68, as best shown in FIG. 11, has four surfaces.

Surface number 1 is a general aspheric. This surface forms an internalimage 56 and serves as the aperture stop for the unit cell train.

It is an important feature of this invention that all of the lighttransmitted through the first surface (which is the entrance pupil)passes through the cell train and out surface number 4 (which serves asthe exit pupil) without vignetting at all angles of scan and for a 4° by4° FOV.

In the embodiment shown in FIG. 11 the surface number 1 is arotationally symmetric surface.

This surface number 1 may also be constructed as a non-rotationallysymmetric surface, such as, for example, a toroid, for operation of thelarge aperture at a look angle which is biased with respect to a normalof the surface of the large aperture 43.

The surface number 2 (as indicated in FIG. 11) is a general asphericsurface and renders the air space 83 telecentric and corrects coma.

The surface number 3 (as indicated in FIG. 11) is a conic aspheric andforms the exit pupil on the surface number 4.

The surface number 4 is a general aspheric. This surface collimates theoutput, corrects spherical aberration and provides the exit pupil (inthe minus one (-1) magnification unit cell train 68 illustrated in FIG.11).

As shown by the wavefront (indicated by the reference numeral 72 inFIGS. 12-16), the collimated output of the light transmitted through theindividual unit cell train 68 is in the same phase for all angles ofscan of the collimating scanner 63.

However, while all of the light transmitted through an individual unitcell train is in phase, the composite or mosaic transmitted wavefront ofthe composite or mosaic beam provided by an array of unit cell trainsand related microlenses can require phase trimming to adjust the stepsin the mosaic wavefront to the nearest integer value of waves in orderto form a good diffraction point spread function at the image on a focalplane.

FIG. 17 is a series of views showing the blur spots produced when lighttransmitted out of the unit cell train shown in FIG. 12 is brought to afocus at the focal plane 47 by the beam shaping lens assembly 45. Theblur spots shown in FIG. 17 are blur spots which are produced by oneindividual unit cell train 68 looking at the different instantaneousfields of view (IFOV) indicated by the respective cross scan and alongscan coordinate locations shown in FIG. 17 with the collimating scannerelement 63 of the unit cell train 68 in the 0° scan position as shown inFIG. 12. The blur spots in FIG. 17 are blur spots produced bymonochromatic light. This FIG. 17 shows the quality of the image over a4° field of view as produced by one cell train at a fixed position (notscanned) of 0°. As can be seen by viewing FIG. 17, all of the blur spotsare produced within the diffraction dark ring 76 as shown in the variousviews. The diffraction dark rings 76 correspond to the first Airy darkring at four micrometers wavelength.

FIG. 18 is a view like FIG. 17 but showing the blur spots produced by anindividual unit cell train 68 positioned at the 20° angle of scan of thecollimating scanner element 63 as shown in FIG. 16.

As can be seen by viewing FIG. 18, the image formation at a full 20° ofinclination is good. Across the center of the field, the image is fullydiffraction limited and would likely remain tight for another degree orso.

FIGS. 17 and 18 demonstrate the high degree of optical correction andimage quality over a substantial (4° by 4°) field of view at every fieldof regard to plus or minus 20°.

FIG. 19 is a cross section view through the two microlens arrays 43A and43B of the embodiment shown in FIG. 10 with the collimating scannerelements 63 at a 0° scan angle. FIG. 19 traces representative raysthrough the individual unit cell trains 68 for a 0° angle of scan andillustrates how the forming of the internal image 56 provides opticallyinactive volumes 59 in the arrays 43A and 43B.

The two microlens arrays 43A and 43B shown in FIGS. 10-19 provide minusone (-1) magnification exit pupils at the exit pupil 67 of each unitcell train.

FIG. 19 also shows how cooling channels 69 are incorporated within theoptically inactive volumes 59 in the array 43A of this particularembodiment of the invention.

FIG. 20 is a computer-generated, isometric view showing the mosaic 85 ofwavefronts 72 just after the exit pupil surfaces 67 of a 9×9 array ofunit cells 68 at the 0° angle of scan shown in FIG. 19. At a 0° scanangle and with monochromatic light the minus one (-1) magnification unitcell trains shown in the FIG. 19 arrays produce a composite wavefront(at the outlet of the large aperture microlens array assembly 43) inwhich there is no offset between the adjacent wavefronts 72 at the exitpupils 67 of the individual unit cell trains.

FIG. 21 is a computer generated, isometric view showing themonochromatic diffraction point spread function at the focal plane 47 ofan imaging telescope in the FIG. 10 embodiment with the array 43B at a0° scan angle as shown in FIG. 19 and for monochromatic light. FIG. 21is the diffraction point spread function corresponding to the image viewshown in FIG. 20.

The view shown in FIG. 20 is the actual wavefront tangent to the exitpupil but just outside the surface number 4 (see FIG. 11) of theindividual unit cell trains. These individual wavefronts 72 are slightlycupped, as illustrated, rather than being completely flat due to a smallresidual spherical aberration in the unit cell train.

FIG. 22 is a view like FIG. 19 but shows the collimating scannermicrolenses 63 in the scanning array 43B positioned at a 20° angle ofscan.

FIG. 22 illustrates how, at this large angle of scan, the lighttransmitted through the individual unit cell trains is imaged at 56within the interior of the wide field static imager 61 to provideoptically inactive volumes 59 which are still sufficiently large topermit the fabrication of the cooling channels 69 without any vignettingwith the optical beams associated with the 20° scan angle.

FIG. 22 also illustrates how, with the minus one (-1) magnificationconstruction, an incoming wavefront (illustrated by the single line 70extending across and normal to all of the ray traces in front of all ofthe entrance pupils 65) becomes offset (as illustrated by the multiplelines 72 normal to the ray traces exiting the individual exit pupils 67)in the multiple segments of the overall composite light beam transmittedthrough all of the individual unit cell trains 68.

The amount of the offset between adjacent wavefronts depends upon thewavelength of the monochromatic light being transmitted through themicrolens arrays. The offsets occur because of the differences 74 in thelengths of the optical paths of the light transmitted through theindividual unit cell trains in the arrays as the arrays are scanned atangles other than 0°.

As noted above phase trimming, after the light passes through the exitpupils 67, is required to restore the offset segments to a steppedwavefront with integer waves per step, if it is required to form gooddiffraction point spread functions in between the aforementioned Eigenangles.

This offset is illustrated in three dimensions in the isometric view ofFIG. 23. FIG. 23 is a computer-generated view like FIG. 20, but showsthe offset of the individual wavefronts 72 transmitted through theindividual exit pupils 67 at a 5° angle of scan as shown in FIG. 13.

FIG. 24 is a view correlated to FIG. 23. FIG. 24 is a computer-generatedview like FIG. 21 but shows how the point spread response is distributedinto a series of spaced peaks of varying magnitude at the focal plane 47for the 5° angle of scan shown in FIG. 13 and for monochromatic light ofa 4.0 micrometer wavelength. This 5° angle of scan was not substantiallyan Eigen angle (or in close proximity to an Eigen angle) for theparticular 4.0 micrometer wavelength of the monochromatic lighttransmitted through the arrays.

FIG. 25 is a computer-generated view like FIG. 20 but shows the offsetsfor the individual wavefronts 72 transmitted through the individual unitcell trains 68 in two 9×9 microlens arrays having the minus one (-1)magnification construction shown in FIG. 10 with a 5° angle of scan (asshown in FIG. 13) and operating with monochromatic light at a wavelengthof 3.775 micrometers.

FIG. 26 is a view correlated with FIG. 25. FIG. 26 is acomputer-generated view like FIG. 21 but showing the monochromaticdiffraction point spread function for the 5° angle of scan shown in FIG.13 and for the monochromatic light at a wavelength of 3.775 micrometers.This 5° angle of scan was substantially an Eigen angle (or in closeproximity to an Eigen angle) for the particular 3.775 micrometerwavelength of the monochromatic light transmitted through the arrays.

FIG. 26 illustrates how a point source can be resolved under theseconditions. However, an extended image (over a substantial field ofview) is not satisfactorily resolved under these conditions with theminus one (-1) magnification.

A comparison of FIG. 23 and FIG. 24 with the respective FIG. 25 and FIG.26 shows that the monochromatic system is very sensitive to wavelengthwith a minus one (-1) magnification. As will be discussed in more detailbelow with particular reference to FIGS. 31 and 32, a plus one (+1)magnification design will improve image resolution for monochromaticlight at Eigen angles for an extended, or substantially extended, FOV.However, for polychromatic light, stepped wavefront correction will berequired for a successful imaging system.

Another embodiment of a unit cell train constructed in accordance withthe present invention is illustrated in FIGS. 27 and 28.

FIG. 28 is a side-elevation view of a microlens array assembly 43 whichincorporates unit cell trains providing plus one (+1) magnification. Theembodiment of the microlens array assembly 43 shown in FIGS. 27 and 28comprises four arrays, arrays 43A, 43B, 43C and 43D, of microlenses(rather than just the two arrays of microlenses shown in the minus one(-1) magnification cell train embodiments of FIGS. 1 and 10).

In FIG. 27 the two arrays 43C and 43D are constructed as back to backarrays and are separated from the respective arrays 43A and 43B by theair Spaces 91 and 93. The arrays 43C and 43D are separated by an airspace 96.

It should be noted that the two arrays 43C and 43D can be replaced by asingle microlens array, to provide the plus one (+1) magnification atthe exit pupil, but the design for a plus one (+1) magnification celltrain is simpler when using a total of four arrays rather than a totalof three arrays.

The unit cell trains produce a first image 56 between the arrays 43A and43C and produce an internal pupil 95 in the air space 96 between thearrays 43C and 43D. This pupil 95 has negative magnification, in thiscase minus one (-1).

The unit cell train construction shown in FIG. 27 produces a secondimage 97 between the arrays 43D and 43B, and the unit cell trainproduces an exit pupil 67 whose magnification is plus one (+1).

In the FIGS. 27 and 28 embodiment, the back to back arrays 43C and 43Dare movable with respect to the outer arrays 43A and 43B to providescanning. Alternatively array 43B could move, by itself, to producescanning. For design purposes it is preferable to move the last array43B rather than both of the internal arrays 43C and 43D.

FIG. 27 shows all of the elements of a unit cell train aligned for a 0°angle of scan.

FIG. 27 traces representative rays through the unit cell train for a 0°field of view.

FIG. 28 is a view like FIG. 27 but traces representative light raysthrough the unit cell trains for a +5° angle FOV and for a -5° angleFOV.

FIG. 29 is a computer-generated view like FIG. 20 but shows the offsetof the individual wavefronts 72 of the individual unit cell trains in a9×9 array of microlenses for a scan position of 3.0 micrometers. Thatis, the scanning arrays 43C and 43D were offset 3.0 micrometers from thecenterline to produce the computer-generated view shown in FIG. 29.

FIG. 30 is a view correlated to FIG. 29. FIG. 30 is a computer-generatedview like FIG. 21 but showing the monochromatic diffraction point spreadfunction corresponding to the offsets in the wavefronts as illustratedin FIG. 29. This particular scanning offset position was not an Eigenposition for the particular wavelength of the monochromatic lighttransmitted through the individual unit cell trains, as shown by thepresence of the two peaks.

FIG. 31 is a computer-generated view like FIG. 20 but showing theoffsets of the individual wavefronts 72 transmitted through theindividual unit cell trains of a 9×9 microlens array having the plus one(+1) magnification unit cell train construction illustrated in FIGS. 27and 28, but for an offset of the arrays 43 and 43D of 9.5 micrometers.

FIG. 32 is a view correlated to FIG. 31. FIG. 32 is a computer-generatedview like FIG. 21 and shows the monochromatic diffraction point spreadfunction for the 9.5 micrometer offset illustrated in FIG. 31. In thiscase, the 9.5 micrometer offset position of the scanner arrays 43C and43D was an Eigen position with respect to the wavelength of themonochromatic light transmitted through the four arrays of the FIG. 27and FIG. 28 embodiment.

This FIG. 32 view illustrates that with a plus one (+1) magnificationarray of unit cell trains an acceptable point spread response can beachieved at Eigen angle positions. The image quality over asubstantially extended FOV is also acceptable at the Eigen anglepositions with the plus one (+1) magnification unit cell train, andwithout the need for phase trimming of monochromatic light.

However, for polychromatic light, as noted above with reference to thecomparison of FIGS. 23-24 with FIGS. 25-26, stepped wavefront correctionis needed in order to produce an acceptable imaging system forpolychromatic light.

FIG. 33 is a view showing another embodiment of the present invention inwhich a telescope 99 is positioned adjacent the entrance side of thelarge aperture formed by the microlens array assembly 43.

The telescope 99, as shown in FIG. 33, reduces the diameter of the lightbeam transmitted through the telescope from the large diameter D at theentrance 101 to the small diameter d at the exit 103 of the telescope.

This enables a smaller size (diameter) of the microlens array assembly43 to be used for viewing a particular field of regard than would bepossible without the telescope. Reducing the size of the microlens arrayassembly 43 facilitates fabrication of the microlens array assembly 43.

However, the overall distance through which the scanning arrays 43B aremoved must be increased, as compared to the distance of movement whichwould be required without the telescope, in order to scan the same fieldof regard.

The angle-aperture product must be preserved.

While we have illustrated and described the preferred embodiments of ourinvention, it is to be understood that these are capable of variationand modification, and we therefore do not wish to be limited to theprecise details set forth, but desire to avail ourselves of such changesand alterations as fall within the purview of the following claims.

We claim:
 1. A unit cell train for transmitting light through twomatched microlenses, one of which is movable with respect to the otherto provide scanning of a field of view, said unit cell traincomprising,a first, wide-field static imager microlens having first andsecond surfaces in the unit cell train, a second, collimating scannermicrolens having third and fourth surfaces in the unit cell train andmovable over an arc with respect to the first microlens to providescanning of the field of view, an air space between the second surfaceand the third surface, the first surface on the first microlens servingas an entrance pupil and aperture stop for the cell train andconstructed to form an image within the first microlens, and the fourthsurface on the second microlens serving as an exit pupil for the celltrain and constructed to collimate all of the light which enters thecell train without reduction of intensity of the light at edges of thefield of view.
 2. The unit cell train defined in claim 1 wherein thefirst surface is constructed as a general aspheric surface.
 3. The unitcell train defined in claim 2 wherein the first surface is arotationally symmetric surface.
 4. The unit cell train defined in claim1 wherein the second surface of each unit cell train is a rear surfaceof the first microlens and is a general aspheric surface.
 5. The unitcell train defined in claim 4 wherein the second, general asphericsurface renders the air space telecentric and corrects coma in the unitcell train.
 6. The unit cell train defined in claim 1 wherein the thirdsurface of the unit cell train is an inlet surface of the secondmicrolens and is a conic, aspheric surface to form an outlet pupil onthe fourth, exit pupil surface of the second microlens.
 7. The unit celltrain defined in claim 6 wherein the fourth, exit pupil outlet surfaceis a general aspheric surface which collimates an output, correctsspherical aberration, and functions as the exit pupil at minus one (-1)magnification.